1. Field of the Invention
The present invention relates generally to acoustic structures that are used to attenuate noise that emanates from a specific source. More particularly, the present invention is directed to providing acoustic structures that have multiple degrees of acoustic freedom.
2. Description of Related Art
It is widely recognized that the best way of dealing with excess noise generated by a specific source is to treat the noise at the source. This is typically accomplished by adding acoustic damping structures (acoustic treatments) to the structure of the noise source. One particularly problematic noise source is the jet engine used on most passenger aircraft. Acoustic treatments are typically incorporated in the engine inlet, nacelle and exhaust structures. These acoustic treatments include acoustic resonators that contain relatively thin acoustic materials or grids that have millions of holes that create acoustic impedance to the sound energy generated by the engine.
Honeycomb has been a popular material for use in aircraft and aerospace vehicles because it is relatively strong and lightweight. For acoustic applications, such as engine nacelles, acoustic materials are added to the honeycomb structure so that the honeycomb cells are acoustically closed at the end located away from the engine and covered with a porous covering at the end located closest to the engine. The closing of the honeycomb cells with acoustic material in this manner creates an acoustic resonator that provides attenuation, damping or suppression of the noise. Acoustic septums are also usually incorporated into the interior of the honeycomb cells in order to provide the resonator with additional noise attenuation properties.
One way of incorporating acoustic septums into the honeycomb cells is to first form planar acoustic inserts from an acoustic material, such as an acoustic mesh or perforated acoustic film. The planar acoustic inserts are made larger than the cell openings. Accordingly, when the inserts are pushed into the cells with a plunger, they are folded into an acoustic septum in the form of a cap. The cap shape provides an anchoring portion that contacts the cell walls and a central septum portion which attenuates the sound waves in the cell. Once inserted into the cells, the friction between the anchoring portion of the acoustic septum cap and the honeycomb walls temporarily locks the acoustic septum cap in place. An adhesive is then used to permanently bond the anchoring portions of the inserted acoustic septum caps to the cell walls.
The permanent bonding of the acoustic septum caps is accomplished by dipping the entire honeycomb into a pool of liquid adhesive. The depth to which the honeycomb is dipped into the adhesive is chosen so that the anchoring portions of the inserted acoustic septum caps are immersed in the liquid adhesive. This adhesive dip process is particularly effective because it provides simultaneous bonding of the many hundreds of acoustic septums that are located within a typical honeycomb acoustic structure.
In many acoustic situations, it is desirable to have honeycomb where the cells have different acoustic damping properties. For example, the depth at which the septum cap is anchored within each cell can be varied to provide cells with different acoustic impedance. These types of acoustic structures are referred to as having multiple degrees of acoustic freedom due to the variability in acoustic impedance of the cells.
The honeycomb that is used in acoustic panels for attenuating noise generated by an aircraft engine is typically from 1 to 3 inches (2.5 to 7.5 cm) thick with cells having cross-sectional areas of from 0.05 to 1 square inch (0.32 to 6.4 cm). The walls of the honeycomb are typically from 0.001 to 0.05 inch thick (0.002 to 0.13 cm). There are a number of challenges associated with anchoring septum caps at different cell depths in the honeycomb cells in order to form an acoustic structure with multiple degrees of acoustic freedom. These challenges depend in part on the desired distance between the various septum cap depths within the honeycomb cells. Locating septum caps at depths within the cell that differ from each other over a relatively wide range, such as from 0.25 to 2 inch (0.64 to 5 cm) presents problems and challenges which are different from the situation where the range of septum cap depths is relatively small, such as from 0.01 to 0.25 inch (0.025 to 0.64 cm).
When the desired difference in septum depths is relatively large, the anchoring portions of the septum caps do not overlap each other. As a result, multiple applications of adhesive are required. For example, when three groups of septa are inserted at three different depths over a relatively wide range, the first group of septum caps must be inserted to the deepest depth and then bonded in place by dipping the honeycomb into the pool of adhesive. Each dipping process leaves a film of adhesive over the entire honeycomb wall that is immersed in the pool of adhesive. After the adhesive for the first group of septum caps has solidified, the second group of septum caps is inserted to the middle depth and bonded in place by dipping again into the pool of liquid adhesive. After the second film of adhesive has solidified, the third group of septum caps is inserted to the shallowest depth and bonded in place by dipping yet again into the pool of liquid adhesive. As a result, three partially overlapping films of adhesive must be applied to the honeycomb walls in order to locate septum caps at three different depths.
The multiple adhesive films that are required to bond acoustic septums at different depths in the honeycomb causes an increase in the weight of the acoustic structure and may even alter the physical properties of the honeycomb. In addition, insertion of the second and third groups of planar acoustic inserts may be difficult due to the buildup of adhesive on the cell walls.
A different challenge is presented when the desired differences in septum cap depths is relatively small. In these situations, the anchoring portions of the different groups of septum caps overlap to some degree so that a single application of adhesive may or may not be sufficient to bond all of the septum caps in place. Instead of reducing the number of adhesive applications, the challenge becomes how to accurately place the septum caps at multiple depths that may differ in placement by only 0.010 to 0.25 inch (0.025 to 0.64 cm).
In some acoustic applications, it may be desirable to locate septum caps within the same honeycomb at cell depths which differ over both a relatively small range and a relatively large range. In these situations, both of the above challenges regarding limiting adhesive applications and accurately placing the septum caps become significant.